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The XMLHttpRequestEventTarget.onloadstart is the function called when an XMLHttpRequest transaction starts transferring data.


XMLHttpRequest.onloadstart = callback;


  • callback is the function to be called when the transaction begins to transfer data.


var xmlhttp = new XMLHttpRequest(),
  method = 'GET',
  url = '';, url, true);
xmlhttp.onloadstart = function () {
  console.log("Download underway);


Specification Status Comment
XMLHttpRequest Living Standard WHATWG living standard

Browser compatibility

We're converting our compatibility data into a machine-readable JSON format. This compatibility table still uses the old format, because we haven't yet converted the data it contains. Find out how you can help!

Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari
Basic support 1 (Yes) 1.0 (1.7 or earlier) 7 (Yes) 1.2
Feature Android Chrome for Android Edge Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support ? 1.0 (Yes) (Yes) ? ? ?


Document Tags and Contributors

 Contributors to this page: erikadoyle, teoli, rolfedh, Sheppy
 Last updated by: erikadoyle,